In this paper, fluid source(s) and processes controlling the chemical composition of VOCs
(Volatile Organic Compounds) in gas discharges from Mt. Etna and Vulcano Island (Sicily,
Italy) were investigated. The main composition of the Etnean and Volcano gas emissions is
produced by mixing, to various degrees, of “magmatic” and “hydrothermal” components.
VOCs are dominated by alkanes, alkenes and aromatics, with minor, though significant,
concentrations of O-, S- and Cl(F)-substituted compounds. The main mechanism for the
production of alkanes is likely related to pyrolysis of organic matter-bearing sediments that
interact with the ascending magmatic fluids. Alkanes are then converted to alkene and
aromatic compounds via catalytic reactions (dehydrogenation and dehydroaromatization,
respectively). Nevertheless, an abiogenic origin for the light hydrocarbons cannot be ruled
out. Oxidative processes of hydrocarbons at relatively high temperatures and oxidizing
conditions, typical of these volcanic-hydrothermal fluids, may explain the production of
alcohols, esters, aldehydes, as well as O- and S-bearing heterocycles. By comparing the concentrations of hydrochlorofluorocarbons (HCFCs) in the fumarolic discharges with
respect to those of background air, it is possible to highlight that they have a geogenic
origin likely due to halogenation of both methane and alkenes. Finally, CFC
(chlorofluorocarbon) abundances appear to be consistent with background air, although the
strong air contamination that affects the Mt. Etna fumaroles may mask a possible geogenic
contribution for these compounds. On the other hand, no CFCs were detected in the
Vulcano gases, which are characterized by low air contribution. Nevertheless, a geogenic
source for these compounds cannot be excluded on the basis of the present data.